Fuel cells membrane

ABSTRACT

A polymer electrolyte membrane (PEM) for fuel cells is provided, as well as a method for manufacturing the PEM by direct casting on the fuel cells electrodes. The PEM, consisting of an ionic liquid entrapped within polysiloxane-RTV matrix, is stable at high temperatures, in acidic and basic environments, and exhibits a high conductivity, without the crossover of methanol.

FIELD OF THE INVENTION

The present invention relates to fuel cell membranes, and moreparticularly to fuel cell membranes which contain ionic liquids.

BACKGROUND

A fuel cell is an electrochemical cell in which the energy of a reactionbetween a fuel, such as hydrogen, and an oxidant, such as oxygen, isconverted directly and continuously into electrical energy. Fuel cellsrepresent an evolving field of technology of pollution-free electricitygeneration that is expected to compete with traditional methods ofcreating and distributing electricity. It is also expected to be used inelectricity powered cars, trucks and buses.

Proton-exchange fuel cells, also known as PEM (Polymer ElectrolyteMembrane) Fuel Cells (“PEMFC”) are low temperature fuel cells which arebeing developed for transport applications as well as for stationaryapplications. In PEMFCs, hydrogen is split at the anode (which inpractice is a thin layer of catalyst on the polymer membrane's surface)into protons, that travel across the membrane to the cathode (similar oridentical to the anode layer) where they combine with oxygen andelectrons (which have traveled to the cathode from the anode via anexternal “load” circuit) to create water, the cell's only product whenusing pure hydrogen. In order to function, the membrane must conducthydrogen ions (protons) but not electrons as this would in effect “shortcircuit” the fuel cell. The amount of water has to be supervised, sinceexcess of water lowers the efficiency of energy production until thepoint of short circuiting, and shortage of water medium or dehydrationof the cell makes the transport of protons from the anode to the cathodedifficult.

The most commonly used polymer electrolyte membrane (PEM) in fuel cellsis perfluorinated polymer containing sulfonic groups called Nafion. Itsspecial structure provides the polymer with high proton conductivity,chemical stability and mechanical strength. However this membranesuffers from several drawbacks comprising: (a) relative high cost; (b)reduced performance in fuel cells due to the high resistivity of theelectrodes/membrane interface; (c) requiring high hydration in order towork effectively; (d) allowing the crossover of methanol from anode tocathode in direct methanol fuel cells (DMFCs) and thus decreasing theperformance of these fuel cells; and (e) limiting the work to acidicenvironment.

DMFCs are fuel cells in which compressed hydrogen is replaced withmethanol for outer energy source replenishment, since methanol is easierto handle than hydrogen and produces energy density orders of magnitudegreater than even highly compressed hydrogen. Overcoming the drawbacksof hydrogen-based fuel cells, which use water as the conducting medium,and Nafion as the membrane, requires the replacement of hydrogen withmethanol and Nafion with another ion-conducting membrane material.

Among the many different proposed membrane alternatives for Nafion is acombination of a polymeric matrix and an ionic liquid. Such compositemembrane is described, for example, in WO 2005/045976 in methanol-basedfuel cells. This publication, however, fails to demonstrate anycontribution of the ionic liquid membrane fuel cells to electricalconduction, especially when compared to cell assemblies where no ionicliquid is used.

It is, therefore, an object of the present invention to provide a PEM(Polymer Electrolyte Membrane) which has high electrolytic conductivity.

It is another object of the present invention to provide a PEM that doesnot depend on the membrane water content and provides high electricalconductivity even at elevated working temperatures.

It is yet another object of the present invention to provide a PEM thatcan operate in acidic as well as in basic environments.

It is yet another object of the present invention to provide a PEMwithout the crossover of methanol through the membrane from the anode tothe cathode.

It is yet another object of the present invention to provide acost-effective low resistivity PEM that can be prepared as aself-standing membrane or as a direct cast on fuel cells electrodes.

It is yet another object of the present invention to provide a methodfor producing a PEM according to the present invention.

This and other objects of the invention shall become clear as thedescription proceeds.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A-G) shows SEM images as well as EDX mapping for the variouselements present in cast polysiloxane based RTV (Room TemperatureVulcanized and the various ILBM (Ionic Liquid Based Membrane) types.ILBM are the initials used throughout the text for the ionic liquidmembranes of the present invention);

FIG. 2 shows the resistance measured as a function of temperature at RH(Relative Humidity)=3% for a type 1 ILBM as compared to the commercialNafion 117 membrane;

FIG. 3 shows the resistance as a function of temperature during aheating-cooling cycle for (a) a type 1 ILBM; (b) a commercial Nafionmembrane;

FIG. 4 is TGA (Thermogravimetric Analysis) and differential TGA curvesobtained for a type 1 ILBM;

FIG. 5 shows the linear sweep voltammograms (scan rate 10 mV/s) obtainedfor oxygen reduction for: (a) a bare Pt electrode (Area=0.07 cm²) and(b) a type 2 ILBM coated Pt electrode (Area=0.07 cm²) in a 1M H₂SO₄solution continuously supplied with air (50 cc/min);

FIG. 6 shows the linear sweep voltammograms (scan rate 10 mV/s) obtainedfor oxygen reduction in a 0.1M KOH solution continuously supplied withair (50 cc/min) for (a) a bare Pt electrode (Area=0.07 cm²), (b) a type2 ILBM coated Pt electrode (Area=0.07 cm²) and (c) a type 1 ILBM (BF₄ ⁻as anion) coated Pt electrode (Area=0.07 cm²);

FIG. 7 shows chronoamperometry measurements at a potential of +0.8 V forthe oxidation of increasing amounts of methanol in 1M H₂SO₄ for: (a) abare Pt electrode (Area=0.07 cm²), and for a Pt electrode (Area=0.07cm²) coated with a type 1 ILBM (CF₃SO₃ ⁻ anion) which is (b) 40 μm and(c) 165 μm thick; and

FIG. 8 shows chronoamperometric currents measured at a potential of +0.8V vs. methanol concentration in 1M H₂SO₄ for: (a) a bare Pt electrode(Area=0.07 cm²), and for a Pt electrode (Area=0.07 cm²) coated with atype 1 ILBM (CF₃SO₃ ⁻ anion) which is (b) 40, (c) 85, (d) 140, and(e)165 μm thick.

SUMMARY OF THE INVENTION

The present invention provides PEMs (Polymer Electrolyte Membranes) thatare comprised of a polysiloxane-based RTV operating as a flexiblenetwork and an ionic liquid, which is entrapped within it. The PEMs ofthe present invention provide chemical stability, high protonconductivity, and improved flexibility.

Since the main properties required for a fuel cell membrane are propermechanical properties and high electrolytic conductivity, the new PEMcomprises a combination of one component, namely the polysiloxane-basedRTV, that imparts to the membrane its mechanical features asflexibility, strength and ruggedness, and a second component, i.e., anionic liquid (IL) present as a salt in its molten state at roomtemperature, that is highly ionically conductive. The abbreviation usedin the present invention for the new membrane is ILBM (Ionic LiquidBased Membrane).

In contrast to Nafion, that has to be pressed onto the electrodes usinghigh temperature and pressure equipment, the new polymer electrolytemembrane (PEM) can be directly cast on fuel cells electrodes. This isexpected to reduce the membrane/electrode interface resistivity and thecost of preparation of the MEA (Membrane Electrode Assembly). Althoughthe ionic liquids which are used in the ILBM are considered to beexpensive, modification of the membranes composition by ion exchangetechniques and in-situ preparation of the ionic liquid in the membranesallow significant cost reduction. For this purpose, the presentinvention proposes the following three types of ILBMs according to themethods of their preparation:

-   1. Membranes in which the proper ionic liquid is introduced during    the preparation of the membrane. Two non-limiting examples are    RTV-BmimBF₄, and RTV-EmimCF₃SO₃ (B=Butyl, E=Ethyl,    mim=methylimidazolium).-   2. Introduction of an ionic liquid with a particular anion in the    membrane and then exchanging it with another anion from its salt in    aqueous solution. For example, the following ion-exchange reaction    is suitable for preparing an ILBM:    (RTV-BmimBF₄)_(membrane)+(NaCF₃SO₃)_(solution)→(RTV-BmimCF₃SO₃)_(membrane)+(NaBF₄)_(solution)-   3. Introduction of an inexpensive solid salt (non-ionic liquid)    after dissolving it in the RTV matrix and then producing the ionic    liquid in this matrix by ion exchange. A non-limiting example for    this route is provided herein:    (BmimCl)_(dissolved in RTV)+(NaCF₃SO₃)_(solution)→(RTV-BmimCF₃SO₃)_(membrane)+(NaCl)_(solution)

In all the above described routes for preparing an ILBM membrane,different derivatives of imidazolium salts, other than those specified,may be used for preparing the ionic liquid either in an already preparedILBM, or by other routes for in situ routes of preparation of themembrane.

In still another embodiment of the present invention pyridinium ionicliquids may be used either for an already prepared ILBM, or pyridiniumsalts may be used for in situ preparation of a pyridinium based ionicliquid.

The presence of the CF₃SO₃ ⁻ group increases the electrolyticconductance of the membranes, and allows for proton transfer, which isnecessary for the membrane application in fuel cells.

In one preferred embodiment the ILBM of the present invention isprepared according to the third method, since it enables the preparationof an ionic-liquid-RTV PEM by using low-cost starting materials in an insitu process. Accordingly, in one preferred embodiment, the presentinvention provides a PEM that comprises a BmimCF₃SO₃ ionic liquidentrapped in a polysiloxane-RTV network membrane, and that is producedin situ by introducing a chloride salt (which is not an ionic liquid butincludes a Bmim cation) in the RTV network and by reacting it with asalt solution of the trifluoro-methyl sulfonate cation. This forms theionic liquid BmimCF₃SO₃ in the RTV matrix.

In one aspect of the present invention, the new membrane can be preparedas a self-standing membrane. In contrast to Nafion which has to bepressed onto the electrodes using high temperature and pressureequipment, the new PEM can be directly cast on fuel cells electrodes.This reduces the membrane/electrode interface resistivity. This is alsoexpected to reduce the cost of preparing the MEA.

In another aspect of the present invention, the high electrolyticconductivity of the ionic liquid, in principle, does not depend onhumidity. Therefore, in contrast to Nafion, the conductivity of the newmembrane does not depend significantly on the water content of themembrane, and therefore can be operational at higher temperatures thanNafion.

In still another aspect of the present invention, the new membrane canbe used in acidic as well as basic media as is demonstrated in half-cellexperiments. This is in contrast to Nafion which can be used only inacidic fuel cells. This is an innovative feature of the PEM of thepresent invention, since efficient commercially available membranes,which can be used in a basic high power fuel cell, are not available.

In still another aspect, PEMs of the present invention essentially donot permeate methanol in DMFCs, particularly, ILBMs with a thicknessabove 140 μm which have been tested in half-cell experiments, andespecially when compared to the commercial 180 μm thick Nafion membranethat does permeate methanol. Methanol crossover through the membranefrom anode to cathode is a main problem encountered when using Nafion inDMFCs (Direct Methanol Fuel Cells). This causes poisoning of the cathodecatalyst and severe decrease of the fuel cell performance. Therefore,the membranes of the present invention are considered to be useful notonly in H₂/O₂ fuel cells but also in DMFCs.

In still another aspect of the present invention, the permeability tomethanol of the ILBM is dependent on the membrane thickness, and on theconcentration of methanol. Particularly, the ILBM of the presentinvention, having a thickness equal to or greater than 140 μm, does notpermeate methanol.

In still another preferred embodiment of the present invention, theionic liquid is dispersed evenly in the membrane matrix.

In one embodiment of the present invention, the ionic liquid is BmimBF₄(1-butyl-3-methylimidazolium tetrafluoroborate, where B stands forbutyl, and mim for methylimidazolium).

In another embodiment of the present invention, the ionic liquid isEmimCF₃SO₃ (1-ethyl-3-methylimidazolium trifluoromethyl sulfonate, whereE stands for ethyl).

In still another aspect of the present invention, the ILBM is resistantto temperature changes, and the ILBM operates also at elevatedtemperatures. The conductivity of the ILBM does not depend significantlyon the membrane water content and it can, therefore, be heated to hightemperatures without substantial loss of conductivity.

In still another embodiment of the present invention, the ILBM ischemically stable up to a temperature of 400° C. according to TGA(Thermogravimetric Analysis) measurements.

In one aspect, the present invention provides a method of preparingILBM, wherein the method comprises combining a first component thatimparts to the membrane its mechanical features, that component being apolysiloxane-based-RTV, and a second component that is highlyconductive, namely an ionic liquid which is in its molten state at roomtemperature.

In one route, the combination of the two abovementioned components takesplace by introducing an ionic liquid to the RTV matrix during thepreparation of the membrane. Preferred combinations prepared this wayare RTV-BmimBF₄ and RTV-EmimCF₃SO₃.

In a second route, the combination of the two components is carried outby introducing an ionic liquid with a particular anion in the membraneand then exchanging it with another anion from its salt in aqueoussolution.

In one preferred embodiment, the following ion-exchange reaction issuitable for preparing an ILBM according to the second route:(RTV-BmimBF₄)_(membrane)+(NaCF₃SO₃)_(solution)→(RTV-BmimCF₃SO₃)_(membrane)+(NaBF₄)_(solution)

In a third route the ILBM of the present invention comprises thecombining of the two components by introducing an inexpensive solid salt(non-ionic liquid) after dissolving it in the RTV matrix and thenproducing the ionic liquid in this matrix by ion exchange. Anon-limiting example for this route is provided herein:(BmimCl)_(dissolved in RTV)+(NaCF₃SO₃)_(solution)→(RTV-BmimCF₃SO₃)_(membrane)+(NaCl)_(solution)

In one preferred embodiment of the present invention, the ionic liquidis homogeneously dispersed in the membrane matrix, wherein the ILBM isprepared in the third route.

In still another embodiment of the present invention, the concentrationof the ionic liquid in the ILBM prepared by the third route is increasedby increasing both the concentration of the initial salt, and the timeof ultrasonic ion-exchange treatment. Particularly, the time ofultrasonic treatment is increased to 3 hours.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1A shows (a) a SEM image for a cross section cast polysiloxane-RTV(also termed silicon-RTV, or RTV) and (b) EDX cross section mapping ofSi for polysiloxane-RTV (magnification: ×300). FIG. 1B shows SEM imagesfor a surface of a type 1 ILBM containing EmimCF₃SO₃ as ionic liquid(a), and EDX surface mapping for fluorine (b), and sulfur (c) elements(magnification: ×250). FIG. 1C shows SEM micrograph for a cross sectionof a type 1 ILBM containing EmimCF₃SO₃ as ionic liquid (a), and EDXcross section mapping for fluorine (b), silicon (c), and sulfur (d)elements (magnification: ×200). It can be concluded from thesemicroscopic observations that the ionic liquid is homogeneouslydistributed within the membrane. However, due to the differentviscosities of the two liquids, RTV and ionic liquid, the latterisolates as bubbles in the RTV matrix. Large drops of the ionic liquidcannot be entrapped by the surrounding matrix which leads to theobservation of some empty bubbles throughout the membrane cross section.

The same phenomenon is observed for a membrane consisting of RTV andBmimBF4 (FIG. 1D) and high porosity due to this effect is seen for type2 ILBM (FIG. 1E). However, bubbles are not observed for RTV+BmimCl,before (FIG. 1F) as well as after exchanging Cl³¹ by CF₃SO₃ ⁻ (type 3ILBM, FIG. 1G).

FIG. 1D shows SEM micrograph for a cross section of a membranecontaining BmimBF₄ as ionic liquid (a) and EDX cross section mapping forsilicon (b), fluorine (c) elements (magnification: ×300).

FIG. 1E shows SEM micrograph for a cross section of a type 2 ILBM(exchange of BF₄ ⁻ by CF₃SO₃ ⁻) (a) and EDX cross section mapping forsilicon (b), fluorine (c), and sulfur (d) elements (maglification:×300).

FIG. 1F is a SEM micrograph of a cross section for cast RTV containingBmimCl (a) and EDX cross section mapping for silicon (b), and chlorine(c) elements, (magnification: ×600).

FIG. 1G is a SEM micrograph of a cross section of a type 3 ILBM(sonication in a NaCF₃SO₃ solution for three hours) (a) and EDX crosssection mapping for silicon (b), fluorine (c), sulfur (d), and chlorine(e) elements (magnification: ×600).

The SEM/EDX results for the different matrixes and membranes aresummarized in Table I below. The conclusions that are drawn from theSEM/EDX experiments are as follows:

-   (a) the highest concentration of CF₃SO₃ ⁻ can be attained in type 1    ILBM which is obtained by mixing RTV and EmimCF₃SO₃ (S/Si=0.34,    compared to 0.05 for type 2 ILBM and 0.005 for type 3 ILBM).-   (b) type 1 and type 2 are more porous than type 3 ILBM.

(c) in type 3 ILBM, the ionic liquid is prepared in-situ after thematrix has been cast. This prevents bubbles formation and the membraneis non-porous, compared to membranes formed by mixing two liquids (types1 and 2 ILBM). The CF₃SO₃ ⁻ concentration in type 3 ILBM can be inceasedby increasing the BmimCl concentration in the matrix and by moreefficient CF₃SO₃ ⁻/Cl⁻ ion exchange (S/Si and S/Cl⁻ are increased by25-30%) by increasing the ultrasonic ion exchange treatment time.Particularly, the time of treatment in the first sonication treatment is1 hour, and is 3 hours in the second sonication treatment. TABLE ISummary of SEM/EDX results for the different matrixes and membranesMatrix/Component (% At/At*) Si S Cl F S/Si S/Cl F/Si RTV 28.64 Type 1ILBM 18.75 6.35 4.10 0.339 0.219 RTV + BMIMBF₄ 25.96 2.71 0.104 Type 2ILBM 30.22 1.58 0.84 0.052 0.028 RTV + BMIMCl 31.5 0.53 Type 3 ILBM34.13 0.36 0.004 0.361 (sonication In CF₃SO₃Na for 1 h) Type 3 ILBM40.44 0.18 0.38 0.005 0.474 (sonication In CF₃SO₃Na for 3 h)*At = atomic percent

FIG. 2 shows the resistance-temperature relationship obtained with atwo-probe configuration for a self-standing ILBM (Type 1, RTV-CF₃SO₃) incomparison to Nafion 117. The thickness of the Nafion membrane is 0.18mm according to the manufacturer, while that of ILBM was estimated inthe 0.5 mm range. The two membranes were tested in similar experimentalconditions (conductivity measured at 25° C. and 3% RH with similarpressure applied on the membrane).

It can be concluded from FIG. 2 that the conductivity of the ILBM is inthe same order of magnitude of Nafion. The calculated specificconductivity at a frequency of 100 KHz of the ILBM at room temperatureand 90% RH is 2.2 mS/cm. However, contrary to Nafion which begins todehydrate at temperatures higher than 65° C., the conductivity of theILBM does not depend substantially on the membrane water content and itcan be heated to high temperatures without significant loss ofconductivity. This can also be deduced from FIG. 3, in which theresistance was followed for both membranes for a whole cycle of heatingup to 100° C. and cooling down to 25° C. Remarkable hystheresis isobserved for Nafion (FIG. 3B) due to almost total hydration at hightemperatures while insignificant increase of resistance is obtained forthe ILBM after the heating half-cycle (FIG. 3A).

FIG. 4 shows a thermogravimetric analysis curve of the ILBM. It isclearly demonstrated by this Figure that the membrane is stable up to atemperature of 400° C. while degradation occurs only at highertemperatures.

Type 2 ILBM were cast as films on Pt electrodes and tested in half-cellsin acidic or basic electrolyte. FIG. 5 shows a linear sweep voltammogramfor oxygen reduction obtained in air saturated 1M H₂SO₄ for a Ptelectrode coated with a type 2 ILBM (curve b) as compared to thatobtained for a bare Pt electrode (curve a). The half wave potential(E_(1/2)) for oxygen reduction at the membrane coated electrode iscathodically shifted by ˜100 mV, while the limiting current increases by˜25%.

FIG. 6 shows linear sweep voltammograms for oxygen reduction at a barePt electrode (curve a) and for type 1 (BF₄ ⁻) (curve c) and type 2(CF₃SO₃ ⁻) (curve b) ILBM-coated Pt electrodes in 0.1M KOH. In thiscase, E_(1/2) for oxygen reduction at the type 2 ILBM-coated electrodeis anodically shifted by ˜50 mV, while the limiting current decreases by˜30% as compared to the bare electrode. However, oxygen reduction at theBF₄ ⁻ based membrane (type 1 ILBM) yields much less satisfactoryresults: E_(1/2) is much more cathodic and currents are significantlylower than those obtained with the CF₃SO₃ ⁻ based membrane (type 2 ILBMin which BF₄ ⁻ was exchanged by CF₃SO₃ ⁻). It can be concluded fromFIGS. 5 and 6 that as opposite to Nafion, the CF₃SO₃ ⁻ based ILBM can beoperated as films/membranes in acidic as well as in basic environments.

The ILBM were also tested for their permeability to methanol. This wasachieved by coating Pt electrodes with type 1 ILBM (CF₃SO₃ ⁻ as anion)films and performing chronoamperometric measurements in 1 M H₂SO₄ at+0.8 V, a potential at which methanol is oxidized at a bare Ptelectrode. FIG. 7 shows typical chronoamperometric curves obtained inthe absence and presence of increasing concentrations of methanol asobtained for a bare Pt electrode (curve a) as well as for Pt electrodescoated with ILBM, 40 and 165 μm thick (curves b and c, respectively).

Since from FIG. 7 it can be deduced that the permeability of methanol inthe ILBM depends on methanol concentration and the membrane thickness,methanol oxidation currents at +0.8 V were measured as a function ofthese two parameters, as summarized in FIG. 8. The slope of current vs.methanol concentration is smaller for the ILBM coated electrodes (curvesb-c) as compared to the bare electrode (curve a). Moreover, this slopedecreases as the membrane thickness increases. Membranes thicker than140 μm do not permeate methanol. This enables to consider the ILBM asgood alternatives to Nafion in DMFCs (Direct Methanol Fuel Cells), sinceNafion 117 (˜180 μm thick) is known to permeate methanol.

Experimental Data

The following details the materials and properties thereof, which wereused in the preparation of the ILBMs of the present invention, andmethods of measurement and analysis employed on the membranes.

RTV is a vulcanized polysiloxane Silastic (R) 9161 RTV Rubber of DowCompany.

The Catalyst used for in situ preparation of the ionic liquid in themembrane is 9162 of Dow Company.

The composition of RTV+catalyst contains the following ingredients:

-   a) Silicic acid (H₄SiO₄), tetraethyl ester, hydrolyzed; CAS. No.    68412-37-3; 47% (w/w).-   b) Tetraethyl silicate; CAS No. 78-10-4; 46% (w/w).-   c) Dibutyltin dilaurate; CAS No. 77-58-7; 7.0% (w/w).-   d) Diluter solution: 200 Fluid, Dow Company, chemical composition:    (CH₃)₃SiO[SiO(CH₃)₂]_(n)Si(CH₃)₃

Ionic Liquids used are BmimBF₄ (1-butyl-3-methylimidazoliumtetrafluoroborate) (Fluka or Chemada) or EmimCF₃SO₃(1-ethyl-3-methylimidazolium trifluoromethyl sulfonate) (Fluka).

Typical type 1 ILBM composition is prepared according to the followingweight relation:

-   1:1:1:1 (weight ratio) of RTV:Diluter:Catalyst solution:Ionic    Liquid. Preparation is carried out by either manual mixing or by    using an electric mixer.

Typical preparation of type 2 ILBM contains the following:

Type 1 BF₄ ⁻ based ILBM which is exposed overnight to a solutioncontaining 1.65 M CF₃SO₃K at pH 13.

Typical preparation of type 3 ILBM is carried out according to thefollowing steps:

First step: mixing of 1:1:0.35:0.70:0.60 (weight ratio) ofRTV:diluter:BmimCl:NaOH (1M):catalyst and letting the mixture overnightfor hardening.

Second step: exposing to a 1M CF₃SO₃Na solution (ultrasonic treatment inthis solution for 1-3 hours and then exposure overnight to thissolution).

Third step: exposure for at least 1 hour in 1M H₂SO₄.

The density of the self-standing membranes was determined (weight ofsamples with known dimensions) to be 1.10±0.05 g/cm³.

The thickness of ILBM films on electrodes was determined by the weightof the films, their density and the area of the electrodes.

All potentials in half-cell experiments refer to Ag/AgCl/KCl (satd.) asreference electrode. Electrochemical reactions were driven by anEcochemie potentiostat.

Resistance was measured with a Wayne Kerr 4265 Automatic LCR meter andapplying a constant pressure of ˜40 N·cm⁻² on the membrane which wasplaced between two graphite electrodes. A heating element was introducedin one of the graphite electrodes and temperature was measured using athermocouple located near the membrane.

Test Methods

Table II below details the different test and measurement methods usedin the analysis of the ILBMs of the present invention. TABLE II Testmethods used in analyzing ILBMs Test Method Description SEM (Scanning Anelectron microscope forms a three-dimensional Electron image on acathode-ray tube by moving a beam of Microscopy) focused electronsacross an object and reading both the electrons scattered by the objectand the secondary electrons produced by it. EDX (Energy Mainly used inchemical analysis, in a X-ray Dispersive fluorescence spectrometer or inan Electron X-ray Microprobe (e.g. inside a scanning electronspectroscopy) microscope): A semiconductor detector, usually silicondoped with lithium (Si(Li) detector) is polarised with a high voltage;when a X-ray photon hits the detector, it creates electron-hole pairsthat drift due to the high voltage. The electric charge is collected,like charging a condensator; the increment of voltage of the condensatoris proportional to the energy of the photon, and it is thus possible todetermine the energy spectrum. The condensator voltage is resetregularly to avoid saturation. TGA Determines changes in weight inrelation to change in (Thermo- temperature. Relies upon a high degree ofprecision gravimetric in three measurements: weight, temperature, andAnalysis) temperature change. Voltammetry A method for measuringelectrical current or potential, in which only a small portion of thematerial is reduced (or less commonly oxidized) electrolytically.Chrono- A method used to study diffusion-controlled amperometryelectrochemical reactions and complex electrochemical mechanisms. It isperformed by applying an initial potential at which no faradaic reactionis occurring, then stepping the potential to a value at which theelectrochemical reaction of interest takes place. The current ismeasured throughout the experiment.

EXAMPLES

The following examples demonstrate in a non-limitative way methods forpreparing the different types of membranes of the present invention.

Example 1 Preparation of Type 1 ILBM Membrane

Self-standing membrane: 120 mg of silicon-RTV (Dow Corning) were weighedand mixed with 120 mg of diluting agent (200 fluid—Dow Corning). To thiswere added 120 mg of ionic liquid, either EmimCF₃SO₃ or BmimBF₄, and 120mg of hardening agent RTV-9162. All ingredients were mixed until ahomogeneous mixture was achieved. 180 mg of the mixture were cast on around teflon plate with a diameter of 2 cm, and air dried for 24 hoursat room temperature to produce a self-standing membrane. The thicknessof the dry membrane, as measured with a micrometer, was 280 μm, and 450μm for RTV-BmimBF₄, and RTV-EmimCF₃SO₃, respectively.

Direct casting of membrane on electrode: 100 mg of the mixture asprepared above for the self-standing membrane was directly cast as athin layer on an aerogel carbon electrode (Marketech) having a diameterof 1.65 cm. Another aerogel electrode, having the same diameter, wasthen placed on the membrane layer before hardening. The MEA (MembraneElectrode Assembly) was allowed to air dry for 24 hours at roomtemperature. The calculated thickness of the membrane according to itsgeometrical surface area, weight and density, was 320 μm and 300 μm forRTV-BmimBF₄, and RTV-EmimCF₃SO₃, respectively.

Example 2 Preparation of Type 2 ILBM Membrane

A self-standing membrane or directly cast MEA of RTV-BmimBF₄ as preparedin Example 1 above, was submerged in a 3 ml basic solution of NaCF₃SO₃(1.65 M of CF₃SO₃H+1 M of aqueous solution of NaOH in a volume ratio of1:1.5; pH=13) and was sonicated for 3 hours, followed by further 16hours without sonication. After ion exchange took place, the membrane orMEA was washed with de-ionized water and air dried for 24 hours at roomtemperature.

Example 3 Preparation of Type 3 ILBM Membrane

105 mg of solid BmimCl (Aldrich), 140 mg of 1 M aqueous solution of KOH(4 drops) for dissolving BmimCl, and 300 mg of a diluting agent (200fluid Dow Corning) were mixed together. 300 mg of RTV 9161 were thenweighed and added together with 8 drops of RTV 9162 hardener. Theingredients were mixed until achieving a homogeneous mixture. 265 mg ofthe final mixture were cast into a round teflon plate having a diameterof 2 cm to produce a self-standing membrane. The cast was air dried atroom temperature for 24 hours. The thickness of this self-standingmembrane, as measured with a micrometer, was 430 μm.

Direct casting type 3 membrane o electrode was achieved by applying 125mg of the mixture on a aerogel carbon electrode with a diameter of 1.65cm, followed by placing a second aerogel carbon electrode, with the samediameter, over the membrane layer before its hardening. The MEA was thenair dried for 24 hours at room temperature.

Ion exchange in the membrane of Cl⁻ to CF₃SO₃ ⁻ was carried out bysubmerging the membrane or MEA in a 3 ml of 1 M aqueous solution ofCF₃SO₃Na (Aldrich) for 3 hours in a sonicator, followed by 16 hourswithout sonication. After ion exchange took place, the membrane or MEAwere washed in de-ionized water and air dried for 24 hours at roomtemperature. The thickness of the self-standing membrane, as measuredwith a micrometer, was 430 μm. The calculated thickness of the MEAaccording to surface area, weight and density of the membrane was 320μm.

While examples of the invention have been described for purposes ofillustration, it will be apparent that many modifications, variationsand adaptations can be carried out by persons skilled in the art,without exceeding the scope of the claims.

1. A PEM (Polymer Electrolyte Membrane) for fuel cells comprisingpolysiloxane-RTV matrix and an ionic liquid, wherein said ionic liquidis entrapped within said polysiloxane-RTV matrix.
 2. A PEM according toclaim 1, wherein said ionic liquid is evenly dispersed in saidpolysiloxane-RTV matrix.
 3. A PEM according to claim 1, wherein said PEMis stable at a temperature of up to about 400° C.
 4. A PEM according toclaim 1, wherein said PEM operates under acidic conditions.
 5. A PEMaccording to claim 1, wherein said PEM operates under basic conditions.6. A PEM according to claim 1, which is essentially impermeable todiffusion of methanol, preferably the thickness of said PEM is about 140μm.
 7. A PEM according to claim 1, wherein said PEM is directly cast onan electrode, or said PEM is self-standing.
 8. A PEM according to claim1, wherein said ionic liquid is an imidazolium based liquid or apyridinium based liquid.
 9. A PEM according to claim 8, wherein saidionic liquid is selected from the group consisting of BmimCF₃SO₃,EmimCF₃SO₃, and BmimBF₄.
 10. A PEM according to claim 1, wherein saidPEM is prepared by mixing a polysiloxane-RTV and an ionic liquid.
 11. APEM according to claim 1, wherein said PEM is prepared by introducing anionic liquid with a first anion into polysiloxane-RTV and thenexchanging said first anion with another anion from its salt in anaqueous solution.
 12. A PEM according to claim 11, wherein said firstanion is tetrafluoroborate, and said second anion is trifluoromethylsulfonate.
 13. A PEM comprising polysiloxane-RTV and ionic liquid,wherein said PEM is prepared by providing a solid salt which isnon-ionic liquid after dissolving in the polysiloxane-RTV matrix andthen producing the ionic liquid in the polysiloxane-RTV matrix by ionexchange.
 14. A PEM according to claim 13, wherein said PEM is directlycast on an electrode
 15. A PEM according to claim 13, wherein said PEMis allowed to be a self-standing membrane.
 16. A PEM according to claim13, wherein said solid salt is an imidazolium based or a pyridiniumbased salt.
 17. A PEM according to claim 16, wherein said solid salt isBmimCl or EmimCl.
 18. A method for preparing a PEM, wherein said methodcomprises the step of combining a polysiloxane-RTV matrix with an ionicliquid component to form an electrically conductive membrane.
 19. Amethod according to claim 18, wherein the combination of thepolysiloxane-RTV component with the ionic liquid is carried out byintroducing said ionic liquid to said polysiloxane-RTV matrix during thepreparation of the membrane.
 20. A method according to claim 18, whereinthe combination of the polysiloxane-RTV component with the ionic liquidis carried out by introducing said ionic liquid with a first anion inthe polysiloxane-RTV component and then exchanging said first anion witha second anion, said second anion is provided in the form of a salt inaqueous solution.
 21. A method according to claim 18, wherein said ionicliquid is an imidazolium based liquid or a pyridinium based ionicliquid.
 22. A method according to claim 20, wherein said first anion isBF₄ ⁻, said second anion is CF₃SO₃ ⁻, said second anion is provided asan aqueous solution of NaCF₃SO₃.
 23. A method according to claim 18,wherein the combination of polysiloxane-RTV matrix with the ionic liquidis carried out by introducing a solid salt which is non-ionic liquidafter dissolving it in the polysiloxane-RTV matrix, followed byproducing the ionic liquid in the polysiloxane-RTV matrix by ionexchange.
 24. A method according to claim 23, wherein the ion exchangeof the first anion of said solid salt to the second anion is carried outby ultrasonic treatment.
 25. A method according to claim 24, wherein theconcentration of said ionic liquid is increased by increasing theconcentration of the solid salt, and by increasing the time of theultrasonic treatment.
 26. A method according to claim 25, wherein saidultrasonic treatment is carried out for about 3 hours.
 27. A methodaccording to claim 26, wherein said ionic salt is an imidazolium basedor a pyridinium based salt.
 28. A method according to claim 27, whereinthe solid salt is BmimCl.
 29. A method according to claim 27, whereinsaid ionic liquid is BmimCF₃SO₃, or EmimCF₃SO₃.
 30. A method accordingto claim 18, wherein the PEM is prepared by directly casting thecombination of polysiloxane-RTV matrix and ionic liquid on an electrode,or wherein the PEM is allowed to be a self-standing membrane.